ANÁLISE DA QUALIDADE NUTRICIONAL DO BIOFILME.

by da Silva, Caroline Fernandes

Abstract (Summary)

Biofilm is defined as a community of microorganisms associated to an organic matrix, which grows up naturally in any submersed surface. The organic matrix presents in its composition polysaccharides, proteins, nucleic acids, among other polymers. In aquaculture, biofilm has been used to increase the productivity of the system and the survival rate. These improvements in the culture system can be attributed to the biochemical composition of biofilm, i.e., the macro and micronutrients provided by microorganisms that favor the growth of the cultivated organisms. To test this hypothesis, this work aimed to analyze the nutritional quality of biofilm, in the absence of shrimp, in three different depths of the water column in different phases of the biofilm formation, and relate it to the microbial community composition of the biofilm. The experiment was carried out in net pens at the Saco do Justino embayment, in the estuary of the Patos Lagoon, RS, Brazil, in the summer of 2005. To propitiate the setting of biofilm, in each net pen six polyethylene nets have been used. These nets present a 1,2 m² area and an opening of mesh of 1mm. They were set vertically and fixed in bamboos for floating. Every 5 days samples of biofilm were collected at the three water depths, surface (40 cm from the bottom), middle (20 cm from the bottom) and bottom (5 cm from the bottom), in order to verify the concentration of chlorophyll-a, dry weight, protein and lipid concentration and to characterize the microorganisms. Simultaneously, the monitoring of the water quality was carried out. Chlorophyll-a varied between 0,14 ?g.cm-² and 3,49 ?g.cm-² in 30 days of experiment, without any significant difference among the depths. The biofilm dry weight varied from 4 mg.cm-² to 24 mg.cm-². Also with no significant difference among depths. Our study suggested that the verification of the biofilm maturity should be carried by the analysis of both chlorophyll-a and the biofilm dry weight, for these measures comprise all the microorganisms that compose the biofilm, photosynthesizing, or not. The microorganism in the biofilm showed the following abundances: bacteria: 91.106 - 302.106 cels.cm-²; cyanobacteria: 22.106 - 89.106 cels.cm-²; centric diatoms: 4.104 - 76.104 cels.cm-²; pennate diatoms: 1.104 - 26.104 cels.cm-²; flagellates: 3.106 - 15.106 cels.cm-² and nematodes: 13 - 2468 cels.cm-² de nematodeos. The protein concentration varied between 0,3 and 2,7 mg.cm-², and lipids from 8,2 to 105 de mg.cm-². When we convert the protein concentration into percent of biofilm dry weight, the highest value, 26%, was observed at the beginning of the experiment. The correlation between the microorganisms and the biofilm biochemical composition indicates that nematodes and centric diatoms are the main source of lipids in the biofilm, while protein was positively related to the presence of filamentous and attached bacteria, but mainly to centric diatoms. This information will allow a better use of biofilm in order to increase the offer of lipids and proteins to the cultivated organisms.